The present invention relates to a process for the preparation of trialkoxysilanes by reacting silicon metal with an alcohol over a copper catalyst, in particular over copper oxide, and the use of copper oxide in a process for the preparation of trialkoxysilanes.
Trialkoxysilanes, composed of a silicon atom to which three alkoxy groups and a hydrogen atom are bonded, are very reactive and unstable. They therefore undergo numerous reactions, such as additions, copolymerizations, copolycondensations and disproportionation reactions with other organic compounds, and a number of very useful substances being obtained. These in turn are used as starting materials for silane coupling reagents, coating materials or heat-resistant finishes or for obtaining monosilanes in high purity for semiconductor applications.
The trialkoxysilanes can be prepared by the direct action of silicon metal with the corresponding alcohols at from 150 to 500xc2x0 C. using copper-containing catalysts (direct synthesis). In general, the copper-containing silicon catalyst material is suspended in an inert, liquid reaction medium and reacted at from 150 to 300xc2x0 C. by passing in liquid or gaseous alcohol to give the desired trialkoxysilanes. Regarding the conversion of silicon metal and the selectivity of the trialkoxysilane relative to the tetraalkoxysilane present as a byproduct, copper(I) chloride has proven to be a particularly suitable catalyst.
M. Okamoto et al., Catalysis Lett. 33 (1995), 421 to 427 relates to an investigation of the reaction of silicon metal with methanol in a silicon fixed-bed reactor over various copper catalysts. The copper catalysts copper(I) oxide, copper(II) oxide, copper(II) acetate, copper(II) formate, copper(II) phthalate, copper(II) oxalate and copper(I) chloride were compared. The highest silicon conversions (88%) and selectivities for trimethoxysilane relative to tetramethoxysilane (98%) were found for copper(I) chloride.
When copper(I) chloride is used, however, hydrochloric acid is formed and necessitates the use of expensive, corrosion-resistant materials for the reactors used. Furthermore, the presence of chloride in the reaction mixture and in the product leads to a reduction in the yield of trialkoxysilane since the secondary reaction of trialkoxysilane with alcohols to give tetraalkoxysilane is catalyzed by chloride. The hydrochloric acid formed when copper(I) chloride is used as a catalyst can, when the alcohol used is methanol, react with the methanol to give methyl chloride and water, with the result that methanol as a starting material is lost for the trialkoxysilane synthesis.
For these reasons, the use of halogen-free catalysts is desirable.
JP-A-05170773 relates to the preparation of trialkoxysilanes by reacting silicon metal with alcohol in the presence of copper alkoxides. Halide-free products are obtained. The selectivity of this reaction is from 91 to 92% but the conversion of silicon is only from 21 to 32.4%.
An increase in the selectivity and in the silicon conversion is achieved in this reaction according to JP-A-06065257 by using a copper alkoxide catalyst in combination with a metal halide. However, the presence of halide in the reaction mixture and in the reaction product has the abovementioned disadvantages.
EP-A-0285133 relates to the preparation of trialkoxysilanes by reacting silicon metal with alcohols, a copper(II) hydroxide catalyst being used. In this reaction, silicon conversions of from about 80 to 90 mol % are achieved and the amount of tetraalkoxysilanes in the reaction mixture is from about 5 to 10 mol %, based on the silicon.
JP-A-10168084 relates to the preparation of trialkoxysilanes by reacting silicon metal and alcohol over a copper(II) oxide catalyst which has a water content of  less than 3000 ppm. In the preparation of triethoxysilane, a trialkoxysilane selectivity coefficient of 85.2 mol % and a silicon conversion of 91% by weight are achieved. However, the low water content of the catalyst used may require a thermal pretreatment of the catalyst and hence an additional reaction step.
It is an object of the present invention to provide a copper catalyst for the trialkoxysilane synthesis, which catalyst permits high selectivity for trialkoxysilane relative to tetraalkoxysilane and high conversions without the presence of halide being necessary, and which is active in the case of water contents of the catalyst in the percent range. Time-consuming preactivation for generating a catalytically active species is to be dispensed with.
We have found that this object is achieved by a process for the preparation of trialkoxysilanes by reacting silicon metal with an alcohol in an inert solvent in the presence of a copper catalyst.
In the novel process, the copper catalyst contains copper(II) oxide having a BET surface area of xe2x89xa710 m2/g.
Preferably, the copper(II) oxide used in the novel process has a BET surface area of from 10 to 50, particularly preferably from 20 to 40, m2/g.
The BET surface area was determined according to DIN 66131.
By means of the novel process, high silicon conversions and very good selectivities for trialkoxysilane in relation to tetraalkoxysilane are achieved. In general, silicon conversions at the end of the reaction of  greater than 75, preferably from 80 to 90, particularly preferably from 82 to 90, mol % are achieved. The silicon conversion is determined according to the following equation: Si[mol] in the product/amount of silicon used.
The selectivity for trialkoxysilane relative to tetraalkoxysilane is in general greater than 80, preferably from 85 to 90, mol %. The selectivity is determined according to the following equation: trialkoxysilane[mol]/(trialkoxysilane[mol]+tetraalkoxysilane[mol]xc2x7100. Activation of the catalyst used according to the invention by reduction or by a thermal treatment at high temperatures is not required.
The water content of the copper(II) oxides used according to the invention may be in general greater than 5% by weight. The water content of the copper(II) oxide catalyst was not found to have any substantial influence on the results of the reaction.
The copper(II) oxide used according to the invention is preferably freshly precipitated copper(II) oxide. Particularly preferably, the freshly precipitated copper(II) oxide essentially comprises acicular platelets having a length of xe2x89xa6200 nm. Preferably, the acicular platelets have a length of from 20 to 200 nm, particularly preferably from 20 to 100 nm. The particle size was determined by means of transmission electron microscopy (TEM).
The particle size distribution of copper(II) oxide used according to the invention is in general very narrow.
In a preferred embodiment, the freshly precipitated copper(II) oxide is obtained by reaction of copper(II) salts with sodium hydroxide solution, KOH or Ca(OH)2 and subsequent drying, copper(II) oxide being obtained as a mixture with foreign salts.
Foreign salts are to be understood as meaning the salts formed as byproducts in the reaction, corresponding to the copper salts used.
Usually, the copper(II) oxide thus prepared has a foreign salt content of in general from 0 to 20, preferably from 0 to 5, % by weight. The foreign salt generally does not interfere with the reaction of silicon metal with alcohol to give the trialkoxysilane. However, it should be noted that, when the copper(II) salts used are copper chlorides, sodium chloride is obtained as a foreign salt, but usually in very minor amounts, which plays no role with regard to corrosion problems.
Preferably used copper(II) salts are copper(II) chloride, CuSO4, CuBr2, Cu(NO3)2, copper acetate, CuCO3, Cu(CN)2 and/or basic copper carbonate, copper(II) chloride and CuSO4 being particularly preferred.
It is possible to control the foreign salt content in the copper(II) catalyst used in the novel process by washing the freshly precipitated copper(II) oxide with a suitable solvent before drying the product. Thus, a product essentially free of foreign salt can be obtained by washing the freshly precipitated copper(II) oxide with water and subsequent drying. Essentially free of foreign salt is to be understood as meaning a foreign salt content of xe2x89xa62, preferably from 0 to 1, particularly preferably from 0 to 0.5, % by weight, based on copper(II) oxide.
The freshly precipitated copper(II) oxide is generally dried over a period of from 0.5 to 8, preferably from 0.5 to 3, hours, particularly preferably from 0.5 to 1 hour. The temperature during drying is in general from room temperature to 150xc2x0 C., preferably from 60 to 120xc2x0 C., particularly preferably from 90 to 110xc2x0 C. The drying can also be carried out under reduced pressure. However, a copper(II) oxide catalyst which is active in the novel process is also obtained when the drying of the freshly precipitated copper(II) oxide is carried out, after washing of the copper(II) oxide with methanol or with another readily volatile solvent, at room temperature by evaporation of the methanol or of the other readily volatile solvent.
The BET surface area and the particle size of the copper(II) oxide catalysts used according to the invention are dependent on the drying temperature and duration of drying and on the concentration of the solutions used and the feed rate and hence precipitation rate.
Table 1 shows the chloride content, the BET surface area and the crystallite size of freshly precipitated copper(II) oxide catalysts used according to the invention (samples 1 to 3) and of technical-grade, commercial copper oxides (samples 4 and 5).
The technical-grade, commercial copper(II) oxide (tenorite) is usually formed by reacting elemental copper with ammonia, ammonium carbonate and air with subsequent introduction of steam or boiling under superatmospheric pressure with evolution of carbon dioxide and ammonia. However, working-up can also be effected by means of strong alkalis which precipitate copper hydroxide in situ, which is converted into copper(II) oxide by boiling the solution.
The data in Table 1 show that the freshly precipitated copper(II) oxide used according to the invention has a substantially greater BET surface area than technical-grade, commercial copper(II) oxide. Furthermore, the BET surface area of freshly precipitated copper(II) oxide washed only with methanol and then dried in air at room temperature by evaporating methanol is greater than that of freshly precipitated copper(II) oxide which was dried at 100xc2x0 C. (samples 2 and 3).